The present invention relates to a heat-resistant cast steel suitable for exhaust equipment members for automobiles, etc., and more particularly to a heat-resistant, austenitic cast steel having excellent high-temperature strength and machinability, and an exhaust equipment member made of such a heat-resistant, austenitic cast steel.
Some of conventional heat-resistant cast iron and heat-resistant cast steel have compositions shown in Table 3 as Comparative Examples. In exhaust equipment members such as exhaust manifolds, turbine housings, etc. for automobiles, heat-resistant cast iron such as high-Si spheroidal graphite cast iron, heat-resistant cast steel such as ferritic cast steel, NI-RESIST cast iron (Ni-Cr-Cu austenitic cast iron) shown in Table 3, etc. are employed because their operating conditions are extremely severe at high temperatures.
Further, attempts have been made to propose various heat-resistant, austenitic cast steels. For instance, Japanese Patent Laid-Open No. 61-87852 discloses a heat-resistant, austenitic cast steel consisting essentially of C, Si, Mn, N, Ni, Cr, V, Nb, Ti, B, W and Fe showing improved creep strength and yield strength. In addition, Japanese Patent Laid-Open No. 61-177352 discloses a heat-resistant, austenitic cast steel consisting essentially of C, Si, Mn, Cr, Ni, Al, Ti, B, Nb and Fe having improved high-temperature and room-temperature properties by choosing particular oxygen content and index of cleanliness of steel. Japanese Patent Publication No. 57-8183 discloses a heat-resistant, austenitic cast Fe-Ni-Cr steel having increased carbon content and containing Nb and Co, thereby showing improved high-temperature strength without suffering from the decrease in high-temperature oxidation resistance.
Among these conventional heat-resistant cast irons and heat-resistant cast steels, for instance, the high-Si spheroidal graphite cast iron is relatively good in a room-temperature strength, but it is poor in a high-temperature strength and an oxidation resistance. Heat-resistant, ferritic cast steel is extremely poor in a high-temperature yield strength at 900.degree. C. or higher. The NI-RESIST cast iron is relatively good in a high-temperature strength up to 900.degree. C., but it is poor in durability at 900.degree. C. or higher. Also, it is expensive because of high Ni content.
Since the heat-resistant, austenitic cast steel disclosed in Japanese Patent Laid-Open No. 61-87852 has a relatively low C content of 0.15 weight % or less, it shows an insufficient high-temperature strength at 900.degree. C. or higher. In addition, since it contains 0.002-0.5 weight % of Ti, harmful non-metallic inclusions may be formed by melting in the atmosphere.
In addition, since the heat-resistant, austenitic cast steel disclosed in Japanese Patent Laid-Open No. 61-177352 contains a large amount of Ni, it may suffer from cracks when used in an atmosphere containing sulfur (S) at a high temperature.
Further, since the heat-resistant, austenitic cast steel disclosed in Japanese Patent Publication No. 57-8183 has a high carbon (C) content, it may become brittle when operated at a high temperature for a long period of time.